The present invention relates to a cell transmitting apparatus and traffic control system suitable, for example, for control of available-bit-rate traffic in a communication network employing the asynchronous transfer mode.
The asynchronous transfer mode (ATM) provides various service categories, so that the user can select the type of service best suited for the type of data to be transmitted (audio data, video data, various other types of data, or a mixture thereof). Available bit rate (ABR) is one of the service categories. In ABR service, a minimum cell rate and a peak cell rate are established for a connection when the connection is set up, and the network tries to provide the best quality of service for as much traffic as it can handle within these limits.
ABR service is managed by the use of resource management (RM) cells, which are circulated between the source and destination nodes of a connection, establishing a feedback control loop between those two nodes. When a forward resource management (FRM) cell is transmitted toward the destination node of the connection, intermediate nodes such as ATM switches may add congestion information. The destination node places this congestion information in a backward resource management (BRM) cell that it sends back toward the source node. As the BRM cell passes through the intermediate nodes, these nodes may now add or modify explicit rate information indicating the cell rates they can allow on the connection. As a result, the source node receives congestion information and explicit rate information, on the basis of which it can control its cell transmission rate so as to avoid cell loss.
In a variation of this scheme, one or more of the intermediate ATM switches acts as a virtual source (VS) and virtual destination (VD), receiving FRM cells, converting them to BRM cells, sending the BRM cells back toward the source node, generating new FRM cells, and sending them toward the destination node. The single feedback loop between the source node and destination node is thereby split up into two or more smaller loops, each of which can react more quickly to changing network conditions.
In both the basic scheme and this variation, the ATM switches respect the minimum and peak cell rates established for each connection. Accordingly, if a connection has a non-zero minimum cell rate, each intermediate ATM switch will manage its bandwidth and other resources so as to support at least the designated minimum cell rate, even when the source node of the connection is transmitting cells at less than the minimum cell rate.
A resulting disadvantage is that a source node having a non-zero minimum cell rate can tie up network resources unnecessarily, thereby reducing the amount of ABR traffic that the network can carry.